Inverters create an alternating current (AC) from a direct current (DC) power source. For example, one type of inverter converts a 12 V DC battery voltage to a 120 V AC voltage that will power loads such as common household appliances. Inverters have long been used in environments such as boats, recreational vehicles, or other remote areas where AC power is not otherwise readily available. Inverters can provide power for items such as refrigerators, microwaves, hair dryers, coffee makers, toasters, and so on. Inverters are generally smaller, lighter and less expensive than built-in generators, and produce no noise, no fossil fuel, no vibrations, and no fumes.
An inverter generally works by using a series of switches to produce an AC waveform from a DC source. An example of an exemplary inverter arrangement is shown in U.S. Pat. No. 4,742,441 to Akerson, which discloses a combined inverter and converter (battery charger) arrangement, and which is hereby incorporated by reference. In battery charger mode, the arrangement disclosed by Akerson operates to convert an AC signal (applied to the terminals that serve as the inverter AC output terminals) to a DC battery that powers the inverter. FIG. 1 of the Akerson patent is reproduced as FIG. 1 of the present application.
Referring to FIG. 1, an inverter circuit 10 is generally shown. A battery 12 and an AC element 14 that is powered by the inverter are shown in a typical application of the circuit. The low voltage 12 V DC side of the inverter includes the battery 12, the primary windings 16a and 16b of a transformer 16, and two switches 18 and 20. The battery 12 is connected between DC ground and the center tap of a transformer 16. Transformer 16 has a primary coil on either side 16a and 16b of its center tap 16c. Secondary coil 16d has a turns ratio N relative to each of the primary windings 16a and 16b. One side of the primary coil 16a is connected through switch 18 to ground. The other side of the primary coil 16b is connected through switch 20 to ground. By altematingly closing switches 18 and 20, an alternating polarity voltage, equal to N times the voltage of the battery 12, is induced in secondary transformer winding 16d.
The remaining circuitry of the inverter comprises the high voltage 120 V AC circuitry that filters the alternating polarity voltage from the secondary winding 16d and provides the filtered AC voltage to the AC element 14. Included on the high voltage 120 V AC circuitry are switching devices 22, 24, 26, and 28 that are connected to the end taps of secondary coil 16d. This array of bi-directional switching devices permits the synchronously controlled operative connection of the secondary coils end taps to lines 30 and 32, connected in turn to input terminals 34a and 34b of inductor 34. By controllably pulse width modulating the sequence closures of switches 22, 24, 26, 28 at a predefined switching frequency and duty cycle, an AC voltage may be impressed across the inductor's input terminals 34a and 34b. An integrated waveform will appear at the output terminals 34c and 34d of the inductor 34. A filter network 36 includes a series connected inductor 38 and capacitor 40, and a series connected inductor 42 and capacitor 44. Through these elements a substantially sinusoidal AC voltage is presented to AC element 14.
Inverters such as those described above must often generate very high currents in the primary windings (such as windings 16a and 16b) of the transformer in order to generate the desired 120 V AC output voltage for the AC element. Depending on the appliances that are being run, it is not uncommon for the desired power output of such inverters to range up to 3,000 watts or more. Further, when operating in the converter (battery charger) mode, devices such as the arrangement disclosed by Akerson can generate current through the primary winding or, windings of ranges up to 200 amps or more. The high current in the primary windings of the inverter generates a significant amount of heat in the primary windings of a conventional wire-wound transformer and an even greater amount of heat in the solid state switches such as switches 18 and 20 that are used to convert the DC battery current into an AC waveform.
It is known in the prior art that the heat generated by various semiconductors, such as power transistors and the like, must be dissipated through an efficient heat sink in order to maintain an acceptable temperature at the transistor junction. Power transistors are packaged in a variety of geometrical configurations; however, the TO-220 package is quite often used. The TO-220 packaging encapsulates the transistor with a relatively thin, rectilinear, molded block or body of electrically nonconductive potting material, such as epoxy or the like. A flat metal mounting tab or plate is also included in and extends from the block to form one side of the package. The metal tab helps dissipate heat from the transistor junction, and is often electrically coupled to one of the transistor pins. The semiconductor so packaged is then normally positioned in a good heat exchange relation with a heat sink to dissipate unwanted heat. A variety of arrangements have existed for mounting such packaged transistors to heat sinks.
One prior art patent that shows a heat sink mounting arrangement for TO-220 type transistors is U.S. Pat. No. 4,707,726 to Tinder. As shown in Tinder, a series of power transistors in TO-220 packages are mounted in a row with their metal tabs extending out from their body portion. The heat sink that is used is an aluminum extrusion having a plurality of parallel cooling fins. A slot or channel is formed in the heat sink between a pair of substantially parallel walls. Once the transistors are lowered into the channel, a spring assembly with fingers pushes the transistors against the wall of the channel. Heat transfer occurs between the transistors and the wall of the heat sink. An electrical insulator is included between the wall and the transistor for electrical insulation, while still allowing heat transfer.
A traditional heat sink configuration, such as that shown in Tinder, in which the heat sink itself is placed adjacent to (but remains electrically insulated from) the switching transistor bodies, is somewhat limiting in the amount of heat that can be dissipated. One of the problems with a heat sink mounting arrangement such as that shown in Tinder is that the simple spring device for holding the transistors does not produce the magnitude of force required for some applications. More specifically, in some instances, 150 to 300 pounds of force must be used to hold the transistors against the heat sink to ensure proper heat transfer.
Also, in inverters where a heat sink configuration such as that shown in Tinder is used, the switching transistor bodies are often coupled directly to the primary transformer wires through the use of a bus bar. The connection points of the transformer wires to the bus bar can generate significant amounts of heat due to the current densities at the connection points and ohmic contact resistance associated with the interconnection of dissimilar metals. This additional heat can hinder the operation of the transistors.
In addition, the installation of transformers in inverters has often required the use of special tools and significant labor. For example, the primary windings of conventional wire-wound transformers used in relatively high power converters are often formed by more than one parallel-connected wire. In many cases, the transformer primary wires require a special crimping tool and significant force for bending and clamping the wires to a conductor of relatively large cross-sectional area that interconnects the transformer to the positive terminal of the battery. Likewise, bending the wires to interconnect the transformer to a relatively distant switching bus bar has also required significant effort and labor.
The present invention is directed to an inverter that overcomes the foregoing and other disadvantages. More specifically, the present invention is directed to an inverter with a transformer-heat sink configuration that simplifies installation while simultaneously providing increased efficiency relative to the dissipation of thermal energy.